I'd start by telling them that astrology provides you with an invaluable tool for understanding yourself more deeply, and helps clarify the unfolding events of your life and to make better decisions. If the beginner is interested in hearing more details, explain that astrology is based on mapping the planets and luminaries (sun and moon) in the sky, and where they fall along the zodiac and with respect to each other, and that those positions and aspects to each other provide a roadmap so to speak for your own path in life. Go on from there by introducing some basic facts about the signs, and maybe get into moon signs, house placements and the ascendent if time permits. After this point you can suggest a book like Steven Forrest's "The Inner Sky", which I personally highly recommend for a first astrology book.